The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to an image forming apparatus having toner concentration control means capable of controlling toner replenishing means which replenishes toner from a toner container to a developing unit in response to the output of toner concentration sensing means which is responsive to the toner concentration of a developer stored in the developing unit.
An apparatus of the kind described has a developing unit in which a developing sleeve, or developer carrier, is disposed. A developer deposited on the developing sleeve has the toner concentration thereof sequentially reduced due to consumption. There has been proposed an image forming apparatus having two toner concentration sensing means, one responsive to the toner concentration of the developer before development and the other responsive to the toner concentration after development. This type of apparatus replenishes toner when the toner concentration after development is lower than a lower limit or stops replenishing it when the toner concentration before development is higher than an upper limit, thereby maintaining the toner concentration of the developer constant. Such an apparatus is taught in, for example, Japanese Patent Laid-Open Publication No. 64-99073.
However, to enhance accurate detection of the consumption and replenishment of the toner, i.e., to maintain the toner concentration of the developer in the developing unit constant with accuracy, the problem with the apparatus described above is that two toner concentration sensing means are necessary and have to be accommodated in the limited space available in the developing unit. This obstructs space, and cost saving and free layout. Moreover, with the conventional apparatus, it is impossible to execute toner concentration control in harmony with the balance of toner while maintaining developing ability constant.